As a passive filter, the cavity filter has excellent anti-interference performance such that it is widely used in, for example, the field of mobile communications. A cavity filter comprises a cavity body and a cover plate, wherein the cavity body comprises a plurality of cavities, each accommodating a resonant tube. Therefore, a cavity filter includes a plurality of resonant tubes.
The resonant tube is irreplaceable in the cavity filter, and forms a key component that affects the performance of the cavity filter. That is why strict requirements need to be satisfied when manufacturing and processing a resonant tube.
Conventionally, invar steel is used as a raw material for making a resonant tube. Invar steel, a special iron-nickel metal alloy material, has an extremely small thermal expansion coefficient (averagely lower than 10-6/° C. under ambient temperature), and meanwhile exhibits excellent plasticity. The invar steel is conventionally manufactured using a melt casting process.
Upon researches and practices on the prior art, the inventor of the present invention observed the fact that the resonant tube for use in a microwave RF device is more complicated in shape and requires higher accuracy in the manufacture, making the preparation and mechanical processing of a resonant tube with invar steel ingot more costly (higher consumption of raw materials) and limiting the manufacturing efficiency. Moreover, invar steel prepared using a melt casting process tends to incur defects such as segregation of alloy components, and coarse and uneven forged objects. All these drawbacks significantly increase the cost in the manufacture of a resonant tube, and compromise its applications in a microwave resonator.
Furthermore, the conventional cold heading process is mainly used for making such parts as a bolt, nut, nail, rivet, and a steel ball, etc. The materials used for a forging blank may be selected from copper, aluminum, carbon steel, alloy steel, stainless steel and titanium alloy, etc., and the material utilization rate may reach up to 80-90%. However, due to requirements with respect to hardness and expansion coefficient, the conventional materials for making a resonant tube material fail to be molded and formed in one step using the cold heading process.